This invention relates to a method for dimensional control during firing of multilayer circuit boards. More particularly, this invention relates to a method of controlling the dimensions of patterned supported multilayer circuit boards during firing.
Ceramic multilayer printed circuit boards have been used for many years for circuits for electrical apparatus, such as mainframe computers. Such printed circuit boards are made by casting glass and/or ceramic powders together with an organic binder into tapes, called green tapes. A conductive metal circuit can be patterned onto the green tape by screen printing for example. Vias are formed in each green tape that are then filled with conductive materials, called via inks, to connect the circuits of the various layers electrically. The green tape layers are then stacked and aligned, pressed together under pressure, and fired to burn off organic residues and sinter the glass, thereby forming a fired ceramic multilayer circuit board.
Originally ceramics such as alumina were used to form the green tape layers, but these ceramics require high firing temperatures, up to 1500xc2x0 C. This necessitates the use of high melting, refractory conductor metals, such as tungsten or molybdenum, to form the circuit patterns because they can withstand these high firing temperatures. More recently, devitrifying glasses have been used to form the green tapes that can be fired at temperatures of 1000xc2x0 C. or less. Multilayer circuit boards made of these glasses can be used with lower melting, more conductive metals, such as silver, gold and copper. However, these glass-based printed circuit boards have the disadvantage that they are not as strong as alumina circuit boards.
Thus even more recently, low firing temperature glasses have been deposited on support substrates made of metal or ceramic to which the glasses will adhere. The support substrate can be made of thermally conductive materials such as nickel, Kovar, Invar, or composites of Kovar or Invar coated with copper for increased conductivity, as well as thermally conductive ceramics such as aluminum nitride, silicon carbide, diamond and the like. These substrates impart added strength to the composite. A bonding glass, such as described in U.S. Pat. No. 5,277,724 to Prabhu, can be used to improve adhesion of the green tape layers to the support substrate. In addition, if chosen correctly, the bonding glass can reduce shrinkage of the green tape with respect to the support substrate in at least the two lateral, x and y, dimensions, and thus all of the shrinkage of the green tape layers during firing occurs in the thickness, or z dimension, only. This in turn reduces problems of alignment of the circuit patterns in the glass layers and alignment of the circuit patterns to the via holes in the support substrate.
Active devices can also be mounted onto the planar surfaces of the fired glass. When it is desired to place active devices into cavities in the fired glass circuit boards, they have been made heretofore by hot pressing a cavity into the green tape using a shaped die and punch assembly. However, during the firing process, the green tapes shrink, not always controllably. When the green tape is laminated, the lamination pressure plastically deforms the resin in the green tape, causing the cavity walls to flow inwardly, with the result that the cavity is smaller than the punch used to make the cavity cutouts. The shrinkage that occurs during firing of the green tape further contributes to misalignment problems and cavity size problems. These problems are reduced when a support substrate is used because shrinkage is controlled in two, x and y, dimensions. However, there is still some residual shrinkage in these two dimensions, up to about 3%, and thus problems of process control and shrinkage have continued.
Thus improved methods of controlling cavity dimensions during the lamination and sintering of supported multilayer glass substrates would be highly desirable.
We have found that by adding a top layer of an inert ceramic material powder, one that does not sinter at the temperatures used for firing the multilayer green tape laminate, mixed with a high proportion of the resin binder employed for the green tape, shrinkage in the lateral dimensions of the green tape laminate is reduced.
This top layer is particularly useful when it is desired to form cavities in the fired glass circuit board into which devices are to be inserted.
To further improve the control of shrinkage for cavities after formation of the cavity in the green tape stack, the cavity can be filled with the inert ceramic green tape material which maintains the dimensional stability of the cavity during subsequent firing.
The inert ceramic green tape material can also be used to maintain dimensional control of ceramic projections from a green tape surface. Glass/ceramic projections are made from the same glass as used to make the green tapes. These projections are formed by punching openings through a layer of the inert ceramic-binder composition, and filling the spaces with glass green tape material. Upon firing, the glass projections adhere to the fired glass or green tape material, and the surrounding inert ceramic green tape material ensures that dimensional stability of the projections are maintained.